1. Field of the Invention
The present invention relates to an improved transmission and reception system for digital television. More particularly the present invention is provided to overcome any fading of signal that may occur between the transmission and reception of a digital terrestrial TV system.
2. Discussion of the Related Art
Any terrestrial TV system must overcome a number of problems in transmitting signals to a receiver. For example, the United States has adopted the Advanced Television System Committee (ATSC) system using eight level vestigial side band (8-VSB) as its digital television standard. Because the VSB system is a single carrier modulation system, it is susceptible to fading caused by multipath transmission and signal attenuation. These effects are well understood and the probability characteristics have been documented. If the fade is deep, wide and long enough in duration, the demodulation system in the TV receiver will lose synchronization and the signal will be lost. Such fading is particularly severe in mobile reception of the signal used in digital television.
Attempts have been made to correct signal fading that is frequency selective by using, for example, equalization techniques. However such techniques can result in degraded performance when fading occurs. Other techniques are not frequency selective.
One such solution to fading that has been presented is a “staggered multicasting” system which redundantly sends data in the digital communication system to avoid the fading characteristics in a particular channel. This system is described in provisional application, Ser. No. 60/(PU 010153) filed Jul. 19, 2001 by the same inventors as the present application. The contents of this provisional application are incorporated herein by reference. This application discloses repeating the data stream at a period approximately equal to or greater than the statistically expected fade period value. A problem remains, however, in how to organize the redundant data in such a system for optimum use.
Techniques are known that can vary the quality of service (QoS) and scalability characteristics of transmitted data. Such techniques are common in internet protocol streaming services, and rely on creating priorities in the network switches. QoS and scalability techniques may be very useful in switched network broadcast systems. Clearly, however, no such switch network is provided in the television broadcast medium. Lost data packets in the television broadcast system are not caused by traffic congestion, as in the internet, but rather by the lossy nature of the wireless channel.
The above mentioned provisional application discloses broadcasting redundant data in order to provide a level of guaranteed service. The level of redundancy provided in the bitstream directly affects the error robustness of the system.
In an audio/video broadcast system, the audio channel is normally protected more robustly than the video channel. That is, the viewer can accept a degraded video signal or even no video signal for a short time period. However losing the audio is more disturbing to the listener. Therefore, a higher QoS level should be placed on the audio channel. Other arrangements of QoS levels may be desired.
The present invention seeks to produce such a beneficial system by utilizing techniques that add additional robustness to a signal component or channel which has a higher perceived importance to the user (e.g. audio vis-a-vis video). For example, the maximum fade duration which is overcome by the redundant data stream can be longer for higher priority data than for lower priority data. The audio fade duration, for example, can be supported for a larger time period than that supported for the video. In this case, this will cause the delay buffer to be larger for the audio channel but since the data rate of audio is relatively small compared to video, it can be buffered for a low cost.
While the detailed description of the current invention below focuses on the details of the 8-VSB system, it must be recognized that the solution of the current invention is equally applicable to any digital broadcast transmission system that is subject to a fading channel environment.